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Income Taxes (Narrative) (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Operating Loss Carryforwards [Line Items]      
Income tax expense (benefit) related to the Tax Act $ 19,017,000    
Uncertain income tax positions 0 $ 249,000 $ 0
Basis in partnerships 13,377,000 22,922,000  
Deferred tax assets 32,611,000 68,902,000  
Interest and penalties expense 0 0 $ 0
Accrued expenses 0 $ 0  
Federal      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforward $ 67,000,000    
Operating loss carryforward, expiration date Dec. 31, 2029    
Minnesota and Nebraska | State      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforward $ 20,200,000    
Deferred tax assets 1,300,000    
Valuation allowance 1,300,000    
Colorado | State      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforward $ 200,000    
Operating loss carryforward, expiration date Dec. 31, 2028